pollymere Once again they're not going to stop you buying it if you have somebody who is under 18 with you. It is only if they suspect you might give the alcohol to somebody under 18 - so if you came up to the till, whacked down a bottle of WKD/can of beer/whatever and turned to your daughter and said 'You can have this with some ice when we get home' or something, then the cashier is obligated by law to refuse to serve you any alcohol. If you just saunter up, put yout stuff on and say nothing/do nothing that indicates the alcohol you are purchasing is going to your 10 year old, you will not be asked for ID or who the alcohol is for. Plus it's not just stopping people buying for kids outside who give them cash - legally they can't allow anyone buying it for a minor - even parents buying it for their kids at home.
They don't generally base it on what the product is either - whether it's alcopops like WKD or cider or scotch - it's if you or anyone with you has given them reason to suspect you're buying it for somebody who needs to be carded to confirm they are over 18 (i.e anyone who looks under 25). They don't ask every parent that comes to their till if they plan on giving the booze to their kids, and the rules are generally more lax on food items with alcohol in (even though some of it contains more booze in it than cider or WKD but there we go).
That's why you didn't get carded or asked - you didn't give the cashier reason to think you would give the alcohol to your 10 year old (also it was a food item).
Also it's not unreasonable. Again, it's the law (stores have a legal obligation to take reasonable measures to ensure alcohol doesn't get bought by or on behalf of those underage) If they have reason to believe an over 25 os going to give that alcohol to somebody under 18 or something they can't be 100% sure isn't 18, they have a legal obligation to ask for ID of all parties they have reason to believe are going to have that alcohol.
Nobody is penalising you for who you are bringing with you to buy alcohol - there are just certain protocols staff must follow if they have reason to believe the alcohol you are purchasing is going to go to anyone under 18.
It's worth noting that if, say a group of 18 year olds came to my till but only one of them was paying for the beer or whatever, but I heard them talking about a party or about them all sharing said item, I would ask them all for ID because I had reason to believe they were all going to drink the beer, and they all looked under 25. If one of them did not have ID, I would have to refuse the sale because I couldn't be 100% sure one of them wasn't underage.
So they're no just targetting parents, it's not a judgement on people bringing their kids to buy booze (honestly they couldn't care less). It's just following the law and company policy (which as I have mentioned before, they legally must have a policy in place to help them keep this law)